One type of high-voltage FET is a heterostructure FET (HFET), also referred to as a heterojunction or high-electron mobility transistor (HEMT). HFETs based on gallium nitride (GaN) and other wide bandgap III-IV direct transitional semiconductor materials, such as silicon carbide (SiC), are advantageously utilized in certain electronic devices due to their superior physical properties over silicon-based devices. For example, GaN and AlGaN/GaN transistors are commonly used in high-speed switching and high-power applications (e.g., power switches and power converters) due to the high electron mobility, high breakdown voltage, and high saturation electron velocity characteristics offered by GaN-based materials and device structures. Due to the HFETs physical properties, HFETs may change states substantially faster than other semiconductor switches that conduct the same currents at the same voltages and the wide bandgap may improve performance of the HFET at elevated temperatures.
GaN-based HFETs devices are typically fabricated by epitaxial growth on substrate semiconductor materials such as silicon, sapphire and silicon carbide formed into a thin disk or wafer. The fabrication steps for forming electronic devices (e.g., transistors) directly in the semiconductor material are frequently referred to as front-end-of-line (FEOL) processing. During FEOL processing of an HFET, the wafer may be moved from various machines to build the various material layers of the device structure. But because GaN is a piezoelectric material, GaN-based HFET devices are susceptible to charge build-up (positive or negative) during FEOL processing. For example, charge build-up may result from the passivation process which involves deposition or growth of dielectric layers on the surface of a semiconductor. Passivation may be utilized to provide electrical stability by isolating the surface of the wafer from electrical and chemical conditions in the environment. For instance, exposure to air during fabrication of the HFET can cause surface reactions such as oxidation to occur which may impact the overall performance of the HFET device.
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